For the past thousand years, the human history of the North Sea has been played out on a stage constructed largely of herring.

I’ve written about Helen Czerski before, when I reviewed her first book, Storm In A Teacup (2017). At that time I mentioned that she is a physicist with a speciality in oceanography. So this one, subtitled How The Ocean Shapes Our World, is about a topic dear to her heart—the world ocean. It’s about physics, certainly, but also biology, chemistry, geology and human history. As with her previous books, there’s a leavening of personal anecdote—from sailing with Hawaiian navigators to carrying out research on the polar drift ice.

I chose my illustrative quotation at the head of this post not because it summarizes what the book’s about, but because it exemplifies how the book works—making connections between disparate areas of knowledge in a way that’s often quirky but always informative. (In this case, the connection between a tiny copepod, Calanus finmarchicus, the herring shoals of the North Sea, the fishing boats that pursued them, and the migratory “herring lassies” who prepared the catch for market once it was landed.)

Czerski organizes the book into several large chapters. “The Nature Of The Sea” introduces the idea of the world-ocean as a machine—it’s driven by the exchange of heat (warming in the tropics, cooling at the poles), and it moves in response. Czerski breaks down this “blue machine” into four components—temperature and salinity create density gradients; liquidity allows the oceans to move in response to those gradients; and the spin of the planet deflects those movements into the huge, slow gyres of the ocean currents.

“The Shape Of Seawater” introduces the different functional parts of the blue machine—the surface of the ocean and its margins, where the bulk of human activity takes place, and its bottom, with its mid-ocean spreading zones, responsible for the movement of continents, and its broad abyssal plains.

“The Anatomy Of The Ocean” describes the layers within the water column, and how life requires both light (abundant in the upper layers, absent at depth), and nutrients (which settle into the depths, leaving the upper layers relatively nutrient-depleted). For life to thrive, then, there must be a source of vertical motion in the water column, like the one created by the Trade Winds in the Pacific, which push surface water away from the west coast of South America, causing deep, cold, nutrient-rich water to rise to the surface along the coastline.

A section entitled “Messengers” discusses the two ways by which information travels through the ocean—light (very limited range) and sound (very long range). “Passengers” deals with stuff that drifts in the ocean—not just the obvious topic of plankton, but dissolved minerals and gases, too. And “Voyagers” is about creatures (like penguins and people) that move through the ocean, and why they do it.

A final chapter, “Future”, gives the usual warning cry about what a mess we’re making of the natural world, and how we need to fix it. These chapters seem to be obligatory, nowadays, but I often wonder if they’re not just preaching to the choir—the readership for this sort of book is probably already on board with everything Czerski has to say.

You might think that’s a pretty dull list of topics, but Czerski makes it all sparkle. Her style is clear, and her wry footnotes are something of a treat in themselves. But it’s the fresh and memorable nature of her anecdotes and descriptions that really stand out for me. Here she is on the topic of warm water:

Water can store a surprising amount of energy as heat. Imagine two grapefruit-sized globes of water side by side, one full of North Pole ocean at about -1.8°C, and one full of Persian Gulf water at about 30°C. The warmer water has more heat energy, and if you could channel all that extra energy into doing something mechanical, you could lift an SUV (weighing nearly two tons) up by about 7 metres, high enough to be level with the top of a two-storey house.

Isn’t that rather marvellous?

And she illustrates the sheer variety of species concealed within the catch-all term “plankton” by asking us to imagine them all enlarged by a factor of 20,000. A medium-sized foraminiferan would now be the size of a Range Rover. A dinoflagellate would come in at Mini Cooper size, diatoms would be the size of watermelons … and so on down to marine viruses the size of grains of dried couscous.

And she tells us about how the oceans contain dissolved chlorofluorocarbons (CFCs), originally released into the atmosphere from refrigerators and aerosols, and now heavily regulated because of the damage they caused to the ozone layer. These gases entered the surface layers of the ocean during their peak usage in the mid-twentieth century, and that pulse of dissolved and unreactive gases has now been carried downwards by the natural turnover of ocean water—which means oceanographers can sample the water, measure the CFC content, and chart the movement of long, slow, deep currents that travel around the world for centuries before resurfacing.

And then there are the Continuous Plankton Recorders, which have been towed behind cargo ships and ocean liners for decades now, sampling seven million nautical miles of ocean water, with all the samples analysed and archived in Plymouth. Who knew?

At times she rivals Carl Sagan or James Burke in making truly striking connections. My favourite is her discussion of coccolithophores, the tiny marine organisms responsible for forming the White Cliffs of Dover. For a series of Royal Institution brief lectures entitled “My Favourite Element”, she used a piece of chalk from the White Cliffs to draw a diagram of a coccolithophore on a blackboard. The coccolithophore was drawn using coccolithophores! She tells that story in more detail in the book, but I’ll leave you with the presentation itself. The video is less than three minutes long, but if you imagine a whole book presented in the same way, you’ll more or less have the idea of Blue Machine.